A surface-coated cutting tool including a coating film on a base material has conventionally been used. In such a surface-coated cutting tool, various improvements in the coating film have been made in order to further enhance wear resistance or chipping resistance.
For example, a surface-coated cutting tool in which a hard coating layer composed of titanium nitride or the like and a composite oxide layer composed of Al, Si, and Ti are stacked to implement a coating film formed on a base material has been proposed (Japanese Patent Laying-Open No. 09-248702 (Patent Document 1)). The surface-coated cutting tool has been stated as excellent in chipping resistance even under severe conditions. On the other hand, as the composite oxide layer is formed in an amorphous state, its hardness is poor and further improvement in wear resistance has been required.
Meanwhile, a surface-coated cutting tool in which an inner layer containing a nitride of Ti or the like and an outer layer composed of aluminum oxide are stacked to implement a coating film formed on a base material has been proposed (Japanese Patent Laying-Open No. 2005-138210 (Patent Document 2)). The surface-coated cutting tool has intended to improve chipping resistance by forming a part of the outer layer as amorphous and absorbing external shock there. Expansion of an amorphous region in the outer layer, however, may lead to poorer wear resistance as in Patent Document 1 above. Therefore, it is considered that the amorphous region is preferably limited only to a part of the outer layer and other regions should be crystalline.
If a region other than the amorphous region in the outer layer is formed as crystalline, crystals different in crystal structure come in contact with each other at the interface between the inner layer and the outer layer. Then, adhesion therebetween becomes poor and disadvantage such as separation between layers may be caused. In order to improve adhesion between the layers, it has been necessary to lower compressive stress of the inner layer or to form another intermediate layer. If compressive stress of the inner layer is lowered, sufficient wear resistance is not obtained during high-speed cutting and formation of another intermediate layer lowers manufacturing efficiency.    Patent Document 1: Japanese Patent Laying-Open No. 09-248702    Patent Document 2: Japanese Patent Laying-Open No. 2005-138210